1) Field of the Invention
The present invention relates to an image observation optical system. To be specific, it relates to an image observation optical system used in an image display apparatus that can be held on the head or face of an observer and that can be attached to a cellular phone or a portable intelligent terminal.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a large image. Also, in accordance with recent popularization of cellular phone and portable intelligent terminal, requirements for large view of graphics or text data on these apparatuses have grown.
As conventional image observation optical systems used in image display apparatuses, there are those as disclosed in Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 7-333551 and Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 8-234137. In such an optical system, a prism whose surface having reflecting action has a rotationally asymmetric shape, such as anamorphic, toric or free curved shape, is employed, to compensate image distortion, curvature of field, and astigmatism.
However, if the image display apparatus is designed to achieve high image definition and wide field angle using such a prism, chromatic aberration of magnification by the prism itself becomes large, to cause the problem of imaging performance degradation.
Therefore, an object of the present invention is to provide an image observation optical system which is made so compact as to be applicable, as an image display apparatus, to a cellular phone or a portable intelligent terminal, and which can achieve high image definition and wide field angle while controlling chromatic aberration of magnification to be small.
An image observation optical system according to the present invention comprises an image display element and an eyepiece optical system which introduces, without forming an intermediate image, an image formed by the image display element into the center position of an eye of an observer so as to allow the observer to observe the image as a virtual image. The eyepiece optical system is configured to bend the optical axis using reflecting surfaces for compact-sizing of the optical system, with the optical axis lying in a plane, with respect to which the optical system is symmetrically formed, and to have an optical element that has an entrance surface, a plurality of reflecting surfaces and an exit surface, wherein at least one of the reflecting surfaces is provided with a volume hologram (HOE).
Here, the refractive index of the optical element is preferably greater than 1.7. It is because an element that exerts the same power with higher refractive index produces smaller amount of aberrations.
Also, according to the present invention, it is preferred that a prism with a positive refracting power and a HOE are provided and that, upon an image point F0 at the midpoint, and, of image points at the end points under a condition without the HOE, an image point Fb showing the larger chromatic aberration of magnification and an image point Fa showing the smaller chromatic aberration of magnification being defined on a segment that is an intersection formed by the plane of symmetry and an image display surface, the following conditions (1) and (2) are simultaneously satisfied:
xe2x88x921 less than xcfx86y(HOE, Fa)/xcfx86y(Total) less than 2xe2x80x83xe2x80x83(1) 
xe2x88x921 less than xcfx86y(HOE, Fb)/xcfx86y(Total) less than 1xe2x80x83xe2x80x83(2) 
where xcfx86y(HOE, Fa) is a y-direction power of the HOE at the image position Fa, xcfx86y(HOE, Fb) is a y-direction power of the HOE at the image position Fb, and xcfx86y(Total) is a y-direction power of the entire system.
Also, according to the present invention, it is preferred that, in a prism optical system having at least two reflecting surfaces, the HOE has one or two plane of symmetry of power, and the plane of symmetry of power coincides with a plane of symmetry of shape of a base on which the HOE is provided.
According to the present invention, an optical system whose optical axis is bent using reflecting surfaces is configured as an eyepiece optical system, to achieve compact-sizing.
A prism with a positive refracting power is used as this optical system.
As stated above, if high image definition and wide field angle is attempted using a prism, the problem of large chromatic aberration of magnification should occur. The present applicants have thought of making a chromatic aberration of magnification derived from a prism canceled by endowing a diffraction element an effect of a chromatic aberration of magnification with inverse tendency. First, the applicants examined a case where compensation of a chromatic aberration of magnification derived from a prism is made using a relief hologram (DOE) as the diffraction element.
A diffraction element is provided, as scarcely having a thickness, on a transparent base. As shown in FIG. 33, for example, its power distribution varies by position on the surface thereof. Since a chromatic aberration of magnification varies according to the variation of power of the diffraction element, if a diffraction element having a power distribution that causes a chromatic aberration of magnification having an inverse tendency to the chromatic aberration of magnification derived from the prism is provided on a predetermined surface of the prism, the chromatic aberration of magnification derived from the prism can be compensated without size increase of the optical system. Also, if the diffraction element is formed integral with the prism, disorder of alignment regarding the prism and the diffraction element does not occur.
Regarding the hologram element, there are two types; i.e. a relief hologram (DOE) and a volume hologram (HOE). The DOE has the property of low selectivity regarding incident angle and low selectivity regarding wavelength. Thus, the DOE diffracts rays with a particular wavelength incident thereon at a particular angle and images them as desired order rays, while diffracting, at a low diffraction efficiency, other rays with different wavelengths incident thereon at different angles and imaging them as undesired order rays. In contrast, the HOE has the property of high selectivity with respect to incident angle and high selectivity with respect to wavelength. Thus, the HOE exclusively diffracts rays with a particular wavelength incident thereon at a particular angle, while transmitting the remaining rays as 0th order rays so that undesired order rays should hardly be imaged.
If mass-productivity is considered, it is efficient to fabricate the DOE by lathing. In this case, the DOE is necessarily shaped rotationally symmetric. Therefore, it cannot compensate a chromatic aberration of magnification derived from a prism having a rotationally asymmetric surface such as a free curved surface as shown in FIG. 34. Also, if the relief hologram is used, undesired order rays (0th order rays, 2nd order rays) are generated, in addition to the desired order rays, to degrade the image and obstruct observation.
Then, the present applicants examined a case where compensation of a chromatic aberration of magnification derived from a prism is made using a reflection-type HOE instead of the DOE. As shown in FIG. 35A, since the HOE can be constructed to have a rotationally asymmetric power distribution, it can be configured to exert an effect to cancel a chromatic aberration of magnification derived from a prism that has a free curved surface. Also, in general, if a HOE is applied to a reflecting surface, 0th order rays alone are generated as undesired order rays. Moreover, if the HOE is disposed on a surface that is directed opposite to the eye of the observer, as shown in FIG. 1, 0th order rays, which are generated only by a small amount, are effused in directions deviated from the eye of the observer, and thus do not obstruct observation. In this way, if a HOE is used as the diffraction element as in the present invention, image blur by undesired order rays are precluded from generation and resultantly a clear image for observation can be obtained.
As shown in FIGS. 35A-35B, a HOE (represented by H in the drawings) is applied to an optical surface B (for example, a reflecting surface) of the prism. Here, the scope of xe2x80x9capplicationxe2x80x9d is not limited to the case where a sheet-type HOE is attached to the surface of the optical system with adhesive, but includes a case where patterns are mechanically engraved on the surface of the optical system, and a case where the refractive index on the surface are varied according to a fringe pattern.
For example, as shown in FIG. 35A, if the HOE is applied to the optical surface B, which is symmetrically shaped with respect to a vertical plane L1, in such a manner that its plane of symmetry of power coincides with L1, the power of the HOE as viewed from the side is asymmetric with respect to any horizontal plane, as shown in FIG. 35C, and the power of the HOE as viewed from the top is symmetric with respect to the vertical plane, as shown in FIG. 35D.
In contrast, in the case of a DOE fabricated by lathing, since its power is symmetric as viewed from the side and as viewed from the top, it cannot compensate a chromatic aberration of magnification derived from an optical member that has a rotationally asymmetric (i.e. asymmetric with respect to at least one of a horizontal plane and a vertical plane) surface.
For example, consideration is made for a case where, in an image observation optical system provided with a decentered prism which compensated decentered aberrations by a free curved surface component as shown in FIG. 34, a color HOE is additionally disposed on a surface thereof. Under this condition, as shown in FIG. 1, a base surface (surface B), on which the HOE is provided, of the prism 3 is modified to have a spherical shape, and the former free curved surface component of the surface B, which compensated decentered aberrations, is decomposed into polynomial XY power components of the color HOE, a free curved surface component of a surface A, and a free curved surface component of a surface C. In this way, it is made possible to maintain the decentered compensation performance of the prism as before, and, in addition, to compensate the chromatic aberration of magnification derived from surface shape of the prism by means of the power of the HOE.
If the power for compensation of the chromatic aberration of magnification derived from the prism is largely allotted to a single HOE, an even larger chromatic aberration of magnification should be generated. To solve this problem, as shown in FIG. 1, an optical element (a wedge prism) 5 constructed with two opposite surfaces that are plane surfaces non-parallel to each other is preferably arranged to cancel dispersion of bundles of rays at the HOE surface by means of the spectral effect of the wedge prism, so that the chromatic aberration of magnification is compensated by the entire optical system. However, if the power for compensation of the chromatic aberration of magnification derived from the prism is too largely allotted to a single HOE, the chromatic aberration of magnification cannot be sufficiently cancelled even by the spectral effect of the wedge prism.
Here, in reference to the drawings, the power of a HOE is explained.
Let us consider a ray from the center of the pupil to each image position (which is termed xe2x80x9cchief rayxe2x80x9d) and calculate the power of each surface at the position of this ray.
For example, in the image observation optical system shown in FIG. 1, the optical axis lies in a plane, with respect to which the prism optical system is symmetric.
In the explanation of the present invention, the direction parallel to the plane of symmetry (i.e. parallel to the sheet of the figure) is defined as Y direction, and the direction that intersects the Y direction on the local coordinate plane of each surface at right angles is defined as X direction.
Also, since the power depends on azimuth, X-direction power and Y-direction power are calculated separately.
Also, in the explanation of the present invention, on a segment that is an intersection formed by the plane of symmetry and the image display surface, an image position at the midpoint is represented by F0, and image positions at the end points are represented by Fa, Fb. If no HOE were included, asymmetry should appear regarding the chromatic aberration of magnification at Fa and Fb by the effect of the decentered optical system. Under this condition, the image position that should show the larger chromatic aberration of magnification is defined as Fb, and the image position that should show the smaller chromatic aberration of magnification is defined as Fa.
Regarding a surface provided with a HOE, the power derived from the shape of the base surface and the power derived from the HOE are separately considered. The power contributed by the HOE alone is calculated for a chief ray at each of the image positions, to obtain xcfx86y(HOE, Fb), xcfx86y(HOE, Fa). In this case, it is preferred that the following conditions (1), (2) are simultaneously satisfied:
xe2x88x921 less than xcfx86y(HOE, Fa)/xcfx86y(Total) less than 2xe2x80x83xe2x80x83(1) 
xe2x88x921 less than xcfx86y(HOE, Fb)/xcfx86y(Total) less than 1xe2x80x83xe2x80x83(2) 
where xcfx86y(HOE, Fa) is a y-direction power of the HOE at the image position Fa, xcfx86y(HOE, Fb) is a y-direction power of the HOE at the image position Fb, and xcfx86y(Total) is a y-direction power of the entire system.
If the lower limit value of each of Conditions (1), (2) is not reached, compensation performance of the HOE regarding the chromatic aberration of magnification is insufficient, and resultantly the chromatic aberration of magnification is undercompensated.
If the upper limit value of each of Conditions (1), (2) is exceeded, compensation performance of the HOE regarding the chromatic aberration of magnification is too large, and resultantly the chromatic aberration of magnification is overcompensated.
In this case, it is much preferred that the following conditions (3), (4) are satisfied:
0 less than xcfx86y(HOE, Fa)/xcfx86y(Total) less than 1xe2x80x83xe2x80x83(3) 
xe2x88x920.5 less than xcfx86y(HOE, Fb)/xcfx86y(Total) less than 0.5xe2x80x83xe2x80x83(4) 
where xcfx86y(HOE, Fa) is a y-direction power of the HOE at the image position Fa, xcfx86y(HOE, Fb) is a y-direction power of the HOE at the image position Fb, and xcfx86y(Total) is a y-direction power of the entire system.
Furthermore, in this case, it is still much preferred that the following conditions (5), (6) are satisfied:
0.005 less than xcfx86y(HOE, Fa)/xcfx86y(Total) less than 0.4xe2x80x83xe2x80x83(5) 
xe2x88x920.2 less than xcfx86y(HOE, Fb)/xcfx86y(Total) less than 0.2xe2x80x83xe2x80x83(6) 
where xcfx86y(HOE, Fa) is a y-direction power of the HOE at the image position Fa, xcfx86y(HOE, Fb) is a y-direction power of the HOE at the image position Fb, and xcfx86y(Total) is a y-direction power of the entire system.
Also, it is desirable that every X-direction power of the HOE is positive in a region inside the effective diameter for rays (i.e. a power in any local region does not become negative).
Since the entire prism has a positive power, it is desirable, for compensation of chromatic aberration of magnification, that the HOE also has positive powers over the entire surface thereof.
Also, according to the present invention, it is preferred that the prism with a positive refracting power has at least one reflecting surface, and that the HOE is formed on a surface of the prism.
Also, according to the present invention, it is preferred that the prism with a positive refracting power has at least one reflecting surface, and that the HOE is configured to have a rotationally asymmetric power so as to compensate rotationally asymmetric chromatic aberration of magnification at a position between the image display element and the eye of the observer.
As the power of a HOE increases, generation of chromatic aberration of magnification tends to increase. If the chromatic aberration of magnification generated by the prism optical system is large and the power for compensating this aberration is largely allotted to a single HOE, the chromatic aberration of magnification derived from the HOE becomes large.
Therefore, according to the present invention, it is preferred that the optical system comprises a prism with a positive refracting power, uses at least two HOEs at positions between the image display element and the exit pupil, and satisfies the following condition (7):
|xcfx86y(HOE, F0)/xcfx86y(Total)|xe2x89xa60.25xe2x80x83xe2x80x83(7) 
where xcfx86y(HOE, F0) is a y-direction power of the HOEs at the image position F0, and xcfx86y(Total) is a y-direction power of an entire system.
If two HOEs are provided as in the present invention, the second HOE can be configured to generate a chromatic aberration of magnification having inverse tendency to the chromatic aberration of magnification generated by the first HOE. Accordingly, the effect of the chromatic aberration of magnification derived from the second HOE can cancel the chromatic aberration of magnification derived from the first HOE.
In this case, it is much preferred that the following condition (8) is satisfied:
|xcfx86y(HOE, F0)/xcfx86y(Total)|xe2x89xa60.10xe2x80x83xe2x80x83(8) 
where xcfx86y(HOE, F0) is a y-direction power of the HOEs at the image position F0, and xcfx86y(Total) is a y-direction power of an entire system.
Furthermore, it is still much preferred that the following condition (9) is satisfied:
|xcfx86y(HOE, F0)/xcfx86y(Total)|xe2x89xa60.025xe2x80x83xe2x80x83(9) 
where xcfx86y(HOE, F0) is a y-direction power of the HOEs at the image position F0, and xcfx86y(Total) is a y-direction power of an entire system.
Also, in order to compensate the chromatic aberration of magnification generated by the HOE, an optical member constructed with two opposite surfaces that are configured as plane surfaces non-parallel to each other, such as a wedge prism, may be incorporated in addition to the prism with a positive refracting power and the HOE. If, as stated above, any additional HOE is provided instead of the wedge prism so that the power for compensation of chromatic aberration of magnification is divided between the plurality of HOEs, space saving and weight reduction can be achieved in comparison with the configuration with the wedge prism. In this case, if all of the plurality of HOEs are applied to surfaces of the prism, further space saving can be achieved.
It is noted that, according to the present invention, the prism with a positive refracting power is a decentered prism, and has an entrance surface via which bundles of rays emergent from the image display element enter the prism, a plurality of reflecting surfaces at which the bundles of rays are reflected inside the prism, and an exit surface via which the bundles of rays exit out of the prism.
Unless at least one reflecting surface is decentered from an axial chief ray, the path of the axial chief ray incident on the reflecting surface coincides with the path of the axial chief ray reflected therefrom and thus the axial chief ray is interrupted in the observation optical system, where the axial chief ray is defined as a ray travelling from the center of the object point, via the center of the pupil through the center of the image surface, as traced in the reverse direction. As a result, a beam of rays with its central portion being interrupted is used for image formation and thus the image becomes dark at its center or image formation completely fails at the center.
Also, in the case where a reflecting surface having a power is decentered from the axial chief ray, it is desirable that at least one of surfaces included in the prism used in the present invention is shaped as a rotationally asymmetric surface. It is particularly preferred that at least one reflecting surface of the prism member is shaped as a rotationally asymmetric surface in view of compensation of aberrations. Also, the power distribution of the HOE used in the present invention is rotationally asymmetric. The base surface on which the HOE is provided may be shaped as any one of a cylindrical surface, a spherical surface, an aspherical surface, an anamorphic surface, a toric surface, a surface that defines only one plane of symmetry, and a plane-symmetric free curved surface.
Also, a rotationally asymmetric surface used in the present invention may be configured as any one of an anamorphic surface, a toric surface, and a free curved surface that defines only one plane of symmetry. Specifically, the surface is preferably configured as a free curved surface that defines only one plane of symmetry.
Also, according to the present invention, it is desirable to cover the HOE with a dust shield member.
A HOE exposed to the outside air would absorb moisture and expand to change its peak wavelength regarding diffraction efficiency, and thus is likely to affect the diffraction power. To prevent this phenomenon, the dust shield member is adopted.
The dust shield member may enclose the entire image observation optical system.
The dust shield member is preferably provided with a case which accommodates the optical system and a transparent cover via which light enters or exits out of the case. In this configuration, upon the transparent cover being made of material such as glass or plastic, a HOE may be provided, inside the dust shield member, on a surface of the transparent cover using it as the base.
This configuration can prevent dust or the like from being visible as an enlarged image, and, in addition, can protect the diffraction element from moisture outside, which could cause expansion of the diffraction element and change of the peak wavelength regarding diffraction.
In this configuration, it is preferred that a wedge-shaped optical member is used as the transparent cover also. Furthermore, it is preferred that the HOE is applied to the wedge-shaped optical member. Alternatively, the HOE may be sandwiched between two wedge-shaped optical members. In this case, the unit may be sealed with adhesive or the like at the faces of the HOE that are not sandwiched between the wedge-shaped optical members.
Also, an image display element, a main frame in which any one of the above-mentioned observation optical system of the present invention is arranged as an eyepiece optical system, and a support member which is constructed to be mounted on the lateral sides of the head of an observer so as to hold the main frame in front of the face of the observer can be combined into a head-mount type image display apparatus.
In this case, the head-mount type image display apparatus may be configured so that the observation optical system and spectacle lenses are integrally arranged in the main frame.
Alternatively, the head-mount type image display apparatus may be configured so that the support member is constructed to achieve removable mount to side frames of spectacles.
Also, a pair of the observation optical systems can be arranged in parallel as left and right systems so as to configure a head-mount type binocular image display apparatus.
This and other objects as well as features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.